It seems that in spite of my dislike of interspecific fostering that my birds have been doing it without without consulting me! Last year I purchased a pair of star finches which produced four consecutive clutches; they faithfully incubated and then let the babies starve to death within 1-2 days of hatching. I suspect that they themselves had been foster-reared and had lost a vital instinct. Instincts in this species appear to be quite strong because two of the nests the stars made from scratch in a dried palm inflorescence I had hung in the corner of the aviary.

This year I purchased another pair of stars, waited for them to mature and in October they too started to make their own nest in a fern against the side of the aviary.  Not wanting to risk losing the nest in a storm I protected it by hanging a piece of perspex (plexi-glass) against the wire.  I then sat back and waited and noticed that at various times one or other of the pair was missing so I presumed they were incubating. I usually can’t help myself looking in nests but because of the way this nest was constructed I couldn’t see what was inside but I could listen and by mid-November I could hear chicks begging for food. I resisted the urge to take a closer look for ten days and then I noticed the parents leaving the nest, not from the front but a side entrance they had pushed through the wall. This gave me the opportunity look in and what a surprise it was as I looked into a familiar luminescent gape – my stars had been cuckolded by Gouldians!

A year waiting and I still hadn’t got it right or at least the birds hadn’t, or so I thought. I pulled all the chicks out and was rewarded by the sight of two young stars in addition to the Gouldian. As you can see from the photo the Gouldian was perhaps 2 days older than it’s nest mates and very greedy. I happened to have a nest of three Gouldian chicks which were a few days older than the star-fostered bird, as shown in the accompanying photo comparing two very differently reared Gouldian chicks. Hoping to increase the chance of fledging the stars but not wanting to lose the fostered Gouldian and I placed it with the other Gouldians. It sort of worked and I managed to fledge only a single star but the 4 gouldians were fine. After rearranging the babies, the star’s parents refused to re-enter the nest for 24h and the weaker of the chicks died. The parents even started looking at other nest sites until they eventually rediscoverd their baby.  One thing I did learn was never look in a star finch nest as they are just too ready to desert.

Cuckolding is a topic I haven’t read about before – usually aviculturists talk about interspecific pairing and hybrid chicks. Mixed collections are visually stunning but their management can be problematic. In my aviary I have another odd pair consisting of a male shaft-tail finch and a female zebra finch but they have only ever produced zebra babies so she really has cuckolded him. From an biological point of view the cuckoldry going on in aviaries does give some insight into the evolution of nest parasitism. There are many nest parasites from quite diverse bird groups (and as a father myself I can see the advantages – just kidding). The most famous of the parasites are the cuckoos (hence cuckoldry) but it is also common amongst the finches of Africa. Indigo birds and whydahs typically parasitise the nests of wax-bills and unlike cuckoos the young don’t deliberately kill their hosts young and mixed broods are common. Being able to discriminate between your own and other’s young has obvious advantages for the host and this is one of the reasons that the characteristic mouth-markings of chicks is thought to have evolved. The method isn’t fool-proof however and if I can tell the difference it amazes me that the birds can’t but it may not matter because imprinting on many fostered birds means that they have a reproductive preference for their host species. In the instance of my Gouldians and stars, I suspect interspecific progeny would be sterile but if such foster reared birds continued to prefer their own kind and also continued to parasitise the nests of other finches they should be able to increase their reproductive potential by continuing to lay without all the effort invested into rearing chicks.

Parasitism brings to the fore another topic; that of fostering – the deliberate introduction, by the keeper, of eggs or young to another birds nest.  This will be the topic of part two of this post.